Selectivity of Various Liquid Substrates Used in Gas Chromatography H. M. TENNEY Esso Research laboratories, Louisiana Division, Esso Standard Oil Co., Baton Rouge, la.
b Gas chromatography can be applied successfully to many problems only if there are at hand an assortment of column packings having wide differences in selectivity. In this study, 18 liquid substrates have been screened as to selectivity toward types of hydrocarbons and oxygenated compounds. The results permit comparison of retention between compound types at various boiling point levels. Three dipropionitriles (oxy, thio, and imino) are very similar and show very high selectivity. At a given boiling point level, alkylbenzenes are retained nearly 20 times as long as paraffins; cyclic hydrocarbons about twice as long as the corresponding acyclic compounds; ketones a bout seven times as long as ethers and 1.5 times as long as esters. The paraffin oil, squalane, i s the best nonselective substrate for hydrocarbons; spread in retention i s about 10%. A polypropylene glycol of 2000 average molecular weight is least selective for oxygenated compounds; spread in retention i s about 30%.
some small structural differences through the use of more selective stationary liquid phases. The composition of the higher molecular weight ranges of hydrocarbons, as well as other organic compound mixtures, often must be regarded largely in terms of molecular types rather than individual compounds, regardless of the analytical tool used. Gas chromatographic methods will serve this end more effectively if there are suitable column packings available having a high degree of selectivity. A selective substrate is one which causes two identically boiling compounds of different molecular type to be eluted a t substantially different rates. Knowledge of the selectivity of possible liquid substrates is essential to the intelligent solution of specific analytical problems. James, Martin, and Smith (3) pointed out its importance in the separation of amines and pyridines.
I80
G
provides the answer to many problems of the organic chemist, and is justly regarded as possibly his most powerful single analytical tool. However, no single stationary liquid phase will solve all problems of organic analysis. Indeed, it is rarely possible to separate all components of any complex mixture by use of a single liquid substrate, even in such comparatively restricted mixtures as the C6 and C7 saturated hydrocarbons. One solution t o such problems (8) is to make separate fractionations on columns of distinctly different liquid substrates. Because interferences are usually different in each case, the resulting data may be combined by simple arithmetical means, to arrive a t a rather complete analysis. Another approach ( 6 ) is to pass a multicomponent fraction from a first system into a second and different system which will resolve those particular components. Another difficult problem is separation of close boiling compounds of the same general molecular class. It is solved only by employing a large number of theoretical plates and exploiting AS CHROMATOGRAPHY
2
ANALYTICAL CHEMISTRY
Keulemans, Kwantes, and Zaal (4) discussed theoretical considerations in the choice of the stationary liquid but concluded that selection by trial and error is often required. Their experimental data illustrate wide differences in selectivity. Purnell (5) discussed the relation between retention volume and saturation vapor pressure of the solute a t the elution temperature, and presented data for three stationary liquid substrates. James and Martin (2) showed the degree of selectivity for two liquid substrates (paraffin wax and benzgldiphenyl) toward various hydrocarbons. Desty and Whyman (1) reported similar data for n-hesatriacontane and benzyldiphenyl. The studj- reported here 1Tas designed to provide a broad picture of degree of selectivity toward hydrocarbon and oxygenated organic classes for a number of liquid substrates, in a form a t least semiquantitatively usable by other
I60
I40
I20
"
;; 100 g 0 z
H
80
60
40
20
RELATIVE
RETENTION ("-PENTANE - 1 )
Figure 1. Selectivity of Convoil-20 (saturated hydrocarbon oil) ( 1 00' C.) tittle selectivity for hydrocarbons, selective for oxygenated compounds
EXPERIMENTAL
The apparatus n-as assembled in this laboratory and differed in no essential respect from conventional gas chromatographic design. A capillary pipet type of sample introduction system ( 7 ) was used. The colunins consisted of
.
-
.
A blend nas prepared for each class, containing from two to five compounds (Tables I and 11). The boiling point spread was such as to leave no question as to the order of elution. As high purity was not essential, the compounds were used as received from commonly known suppliers of organic chemicals. The hydrocarbons were API standards, n hen not othern ise available. A column length of 1 meter was chosen to permit each blend to be eluted in a reasonable length of time. Eighteen liquid substrates are included in the study (Table 111), though all test blends !!-ere not included in every case. The liquid was incorporated with the firebrick in a m-eight ratio of 3 to 10 or 4 to 10. Retentions are expressed relative to a reference compound which was run frequently enough to compensate for
U-shaped l/Anch metal tubing, contained in a thermostatically controlled, electrically heated oven with circulating air bath. The inert material used as column packing n-as a 30-60 mesh C-22 firebrick. A thermistor-type detector cell m-as used m-ith conventional control circuitry and recording. Helium was the carrier gas in all cases. Ten classes of hydrocarbon compounds and nine oxygenated compound classes 5-ere included in the study.
workers. It was designed as a screening procedure t o be carried out Fvith a minimum expenditure of time.
._
160
I40
i
100
-- -~
- -
I20
_ ~ _ _ ~
__.
_ I _
I
BO--
6
-
IOC
P
:
80
60
40
GI
02
03
05 07
20
IO
30
S O 7 0 IO
RELATIVE RETENTION (n-PEIUTANE
.
20
30 40
70 100
I ]
2
Figure 3.
Table I.
C. 36.1 68.7 98.4
2-Methylparaffins Isopentane 2-Methylpentane
125.7
2-Methylheptane
30 64 94 121 147 80
iio,6
136.2 159.2 183.3
5
7
3
2
3
5
7
L
2-Meth ylhexane
!.P.,
C. 28 GO
90 118
Type 111 Olefinsb 2-xkthyl-l-butene 2-Methyl-1-pentene 2-Methyl-1-hexene
31 60 92 2,4,4-Triniethyl-l-pentene 101
Acetylenes 1-Hexyne 1-Heptyne 1-Octyne
-
1
2030
w - c m
!
,
,
1 om
5%
200
n "EN-ANE.
Selectivity of /3,/3'-oxydipropionitrile at
67" C.
Components in Hydrocarbon Test Mixtures
!.'.,
151
3
P E L A ~ I " E P i - E U T 31 '
_- l o o 3 c. _ _ 150' - C.
n-Paraffins n-Pentane n-Hexane n-Heptane n-Octane n-Nonane Type I olefins. 1-I'entene 1-Hesene 1-Heptene 1-Octene 1-Xonene Alkyl benzenes Benzene Toluene E thylbenz ene n-Propylbenzene n-Butylbenzene
I
I
Figure 2. Characterization of squalane as gas chromatoaraDhic substrate
71
100 126
2 7 0 4
Cyclopentanes Cyclopentane Methylcyclopentane Ethylcyclopentane n-Propylcyclopentane n-Butylcyclopentane
B.P., C. Cyclohexanes 49 Cyclohexane 72 Rlethylcyclohexane 104 Ethylcyclohexane 131 n-Propylcyclohexane 157 n-Butylcyclohe\ane
O
Cyclo-olefins Cyclopentene Cyclohexene 1-Methyl-1-cyclohexene 4-Ethyl-1-cycloheuene 3,5,5-Trimethyl-l-cyclohexene Diolefins Isoprene 1,j-Hexadiene 2,5-Dimethyl-1,5-hexadiene
B.P , "C 81 101 132 157
181
44 3 83 0 110 133 139 34 1 59 5
114
2,3,3,4-Tetramethyl-1,4-pentadiene 129
VOL. 30, NO. 1, JANUARY 1958
a
3
minor drift in operating conditions. n-Pentane was chosen as a substance having reasonably short retention time without seriously detracting from precision of measurement; in some cases its retention time was excessively short. The relative retention (ratio of retention for solute to that of the reference) is independent of column length, flow rate, and ratio of liquid substrate t o solid support, but not independent of operating temperature. Most tests were made a t 100' C. I n a few cases lower temperatures were chosen because of the volatility of the liquid substrate. The glycols were run at 120' C. Measurements of retention were made from an air peak in all cases
to correct for the free space in the system. DISCUSSION OF RESULTS
Data derived from studies of this nature (1, 2, 6) usually have been presented graphically, with the logarithm of the relative retention for each molecular type plotted against the carbon number, the boiling point, or the logarithm of the vapor pressure a t the operating temperature. I n practice, much more is usually known about the boiling range of the sample than about its carbon number or vapor pressure. For that reason,
-
RELATIVE
RELATIVE RETENTION ( n - PENTANE I )
Figure 4.
(loooC.)
4
c.
Primary n-alcohols Methanol Ethyl alcohol Propvl alcohol But$ alcohol Aldehydes Acetaldehyde Propionaldehyde n-Butyraldehyde n-Valeraldehyde
20.2 48.0 76.0 101
hcetates Ethyl acetate n-Propyl acetate n-Butyl acetate
77 102 125
ANALYTICAL CHEMISTRY
O
65 78 98 117
II.
(n-PENTANE = I )
2-ethyl
hexyl
Moderate selectivity for oxy compounds
Moderate selectivity for hydrocarbons
B.P.,
RETENTION
Figure 5. Selectivity of sebacate at 100' C.
Selectivity of 2-ethyl hexyl sebacate
Table
graphical presentation using boiling point has been used in this study. I n Figures 1 to 10 boiling point is plotted us. logarithm retention for eight substrates. I n the cases of some other substrates, however, the lines fall very close together and the resulting plots become very confusing. I n these cases, a better comparison is afforded by reading from the graphs the relative retention for each compound type at three boiling point levels. (Table IV). Actually, the ratio of retentions between different types does not differ greatly with boiling point level. Convoil-20 and @, @'-oxydipropionitrile are included in
Components in Test Mixtures of Oxygenated Compounds
Secondary alcohols Isopropyl alcohol sec-Butyl alcohol Ketones Acetone Methyl ethyl ketone Methyl-n-propyl ketone Methyl isobutyl ketone Methyl-n-amyl ketone Ethers Diethyl ether Diiso ro yl ether E t h y f d u t y l ether Di-n-butyl ether
B.P.,
c.
83 100
57 80 102 118
Tertiary alcohols tert-Butyl alcohol tert-Amyl alcohol Formates Ethyl formate n-Propyl formate Isobutyl formate
B.P., O
c.
83 102
53.5
81.3 98
150 34.6 69
91 142
Acetals 1,l-Dimethoxymethane 1,l-Dimethoxyethane 1,l-Diethoxymethane 1,l-Diethoxp propane
42 64 102 124
both forms of presentation. The latter represent two extremes of selectivity for those substrates extensively tested, particularly as concerns hydrocarbon types. Neither the relative retention plots nor the tabulated data taken from them reveal anything about the actual retention time or volume. To provide this information, the observed retention
volumes for the reference compound, n-pentane, are given in Table V. These vary over a wide range.
Separation of Hydrocarbons.
RELATIVE RETENTICN (n-PENTANE-I)
Figure 6.
RELATIVE
Figure 7.
Selectivity of Convachlor-12 at 100' C. Moderate selectivity for hydrocarbons
Table 111.
Substrate Convoil-20 Squalane 2-Ethyl hexyl sebacate 8,8'-Oxydipropionitrile B,@'-Iminodipropionitrile @'-Thiodipropionitrile Convachlor-12 Perfluorotributylamine Fluorolube-S Polyglycol P-400 Polyglycol P-2000 POlyglyCOl 166-450 Polyglycol 174-500 Trimetacresyl hosphate Silicone oil D.8. 703 Diphenylformamide Apiezon-L Tide
At
times a completely nonselective system is needed. From such a system, hydrocarbons will emerge in t h e order of boiling point without regard t o molecular structure. Satu-
rated hydrocarbons oils would be expected t o be nonselective. Squalane (2,6,10,15,19,23 - hexamethyltetracosane) has been found best of three hydrocarbon materials tested. At the 100" C. boiling point level, the ratio of alkylbenzene to n-paraffin retention is 1.1 t o 1, compared t o 1.25 for Convoil-20 and 1.4 for Apieeon-L. Figure 2
RETENTION (n-PENTANE - 1 )
Selectivity of Convachlor-12 ( 1 00'
C.)
Relatively selective for oxygenated compounds
Materials Studied as Stationary liquid Substrates
Character of Material Saturated hydrocarbon oil (mol. wt. 400) Rochester Division, Consolidated Electrodynamics Corp. Branched chain-(& paraffin ...
Test Mixtures Used Hydrocarbons Oxycompounds X X Partial X
... Chlorinated oil (mol: wt. 326), Rochesfer Division, Consolidated Electrodynamics Corp.
X
..
...
Polymer of trifluorovinyl chloride, av. mol. wt. 775, Hooker Electro-Chemical Co. Polypro ylene glycol, av. mol. wt. 400, Dow ghemical Co. As above, av. mol. wt. 2000 Polyepichlorohydrin, av. mol. wt. 450, Dow Chemical Co. Polystyrene oxide, av. mol. wt. 500, Dow Chemical Co. ... (Mol. wt. 570)
x X X x-_ .. X
X X X X TT
Saturated hydrocarbon lubricant Commercial detergent, containing alkyl aryl sulfonate, Procter & Gamble
A
Partial X
VOL. 30, NO. 1, JANUARY 1958
0
5
shows relative retentions for these two types on squalane a t both 100' and 150" C. It is rather apparent from Figure 3 that p, @'-oxydipropionitrile is highly selective for hydrocarbons. The corresponding thio and imino compounds are almost identical in performance. The aromatics have retentions 15 to 20 times that of the equivalent boiling paraffin. This means that a type separation may be made on a rather wide
boiling fraction. Figure 3 indicates that paraffins boiling up to about210"C. emerge prior to benzene itself. This suggests the use of this substrate for aromatic-nonaromatic separation for nearly the entire gasoline fraction. Actually, a similar compound of lower volatility might prove better in practice. For other than straight-run gasolines. two fractions based on volatility would be required. Such fractions could, of course, be derived by chroma-
Table IV.
= 1) of Various Compound Types at Three Boiling Point Levels for Various Liquid
Relative Retentions (n-Pentane
tographic methods using a nonselective substrate. The perfluorotributylamine (Figure 8) is the only material tested in which cycloparaffins are eluted prior to nparaffins. Isoparaffins are eluted prior to n-parafins over this substrate as well as the dipropionitriles. The difference is small, however. Table VI lists the best substrates based upon this study for specific hydrocarbon analytical separations. From
Substrates
Substrate Operat'ing temp., C. Boiling point level, C. Hydrocarbon types n-paraffins 2-Methyl paraffins Type I olefins Type I11 olefins Cyclopentanes Cyclohexanes Cyclo-olefins Diolefins Acetylenes Blkylbenzenes Oxygenated types Alcohols Primary normal Secondary Tertiary Ketones Ethers Esters Formates Acetates Aldehydes .4ceta1s O
Convoil-20* 60 1.80 1.75 1.90 1.90 2.33
..
,
2.5
1.50
100 100
2-10
5.6 5.4 5.6 5.6 6.7 7.0 7.2
17.0 16.5 17.5 17.5 19.8 20.0 20.5
Osydipropionitrileb 60 1.4 1.4 2.9 3.5 3.1
...
6.4 5 4 12h
67 100 3.2
3.0
...
0.2gh 0.2h 0.28h 0.72 1.35
1 . 4 10* 1.9 . . 2.6 3 . 1 12.0 5 . 4 22
40 100 24h 85 20 77 56 115 7 . 6 15
1.0 1.0 1.0
3 . 3 13* 3 . 3 13h 3 . 3 13h
37 37 40 23
. . . . . . . . .
8.8 ,..-
6 . 5 14.8 6.9 ... 6 . 1 13.0 6.2 14.5 13.7 3 0 . 0 11.5 24h 25.5 60h 57 135
4.5 15.0 6 . 7 19.8
...
140
77 77 95 39
.. , . .. 270 30 190 190
...
65h
Polypropyleneglycol: 40OC 60 1.60 1.50 1.90 1.90 2.30
...
2.90
120 100 140 3.9
3.6
4.5 4.5 5.1 5.1 6.3
9.80 9.Oh
11.3 11.3 12.3 12.3 13.8
Same as naphthenes
3.1
...
7 . 5 18.5 9 . 7 22.7
3.5 2.5h 2.3h 3.4 2.0
10.0 . . . 10.5 . . . 10.0 8 . 7 25:0 5 . 2 14.0
Same as ketones Same as ketones Same as ketones Sameas ketones Silicone D.C. 703
Polystyrene oxidef Tricresylphosphate Substrate 100 120 100 Operating temp., "C. Boiling point level, "C. 60 100 140 60 100 140 60 100 140 Hydrocarbon types 1.67 4 . 3 13.5 1.50 3 . 4 8 . 6 1.65 4.6 13.5 n-Paraffins 1.50 3.4 8 . 6 1.60 4.2 _.. 2-Xethvl Daraffins 1.60 4 . 3 11.5 2.00 4 . 4 1 1 . 5 2.25 5 . 8 16.5 Type I &&ns 1.90 5 . 2 1 5 . 4 1.90 5 . 2 1 5 . 4 2.00 4 . 4 1 1 . 5 2.50 5 . 7 . . . Type I11 olefins 2.60 5 . 3 1 3 . 5 2.25 5 . 8 16.0 Cyclopentanes 2.37 6 . 0 16.5 2.60 5 , 3 13.5 . . . 6 . 7 18.3 . . . 6 . 0 16.5 Cyclohexanes 2.80 7 . 9 17.0 4.00 9 . 2 21.0 Cyclo-olefins 3.9 9 . 7 23 2.60 5 . 3 13.5 2.6 7 . 1 18.3 2 10 6 0 16.5h Diolefins 2.60 5 . 3 1 3 . 5 2.2h 6 4 lgA 3 . 6 h 9 . 7 28 Acetylenes . . 7 9 23.0 . . . 1 4 . 6 33 5 . . . 1 6 . 4 43.5 -4lkylbenzenes Oxygenated types Alcohols 0.76h 2 . 6 ... Primary normal 4h 1 1 . 7 36h 3.3h 1 1 . 5 . . . 3.0 . . . 2.7h 11.5 . . . 2.2h 1 1 . 5 Secondary 2.2h 1 1 . 5 . . . 3.6 2.5h 10.5 . . . Tertiary 1 . 8 5 6 . 0 19:7 4 . 8 1 3 . 5 37.5 Ketones 5 . 6 1 4 . 0 39.0 2.1 6 . 4 18 Ethers 3.2 7 . 2 15.7 . . . . . . I'sters 2.0 6.2 Formates 5 . 0 12.0 4.6 1 2 . 5 43h 2.3h 6 . 9 25h' 5.0 13.0 3 6 . 0 4 . 6 1 2 . 5 43 Acetates ... 2.05 6 . 2 5 . 6 1 4 . 0 39.0 5 . 2 16.3 . . . Aldehydes . . . . . . . . . -4cetals 5.0 9 . 4 19.0 . . . . . . . . . a A saturated hydrocarbon oil, Rochester Division, Consolidated Electrodynamics Corp. PIP'-Oxydipropionitrile (imino and thio compounds show essentially same selectivity). llow polyglycol P-400 (av. mol. wt. 400). Dow polyglycol P-2000 (av. mol. wt. 2000). e Dow polyglycol 166-450 (av. mol. wt. 450). Dow glycol 174-500 (av. mol. wt. 500). 0 Commercial detergent, Procter and Gamble. ,i Extrapolated value.
6
ANALYTICAL CHEMISTRY
Polypropyleneglycol-2000d
Polyepichlorohydrin'
120 100
140
60
1.60 3 . 8
9.6
1.5 1.5 2.1 2.1 2.5 ... 3.7 2.8 3.7
60
i.47 3 . 3
6.9h
1 . 8 0 4 . 4 11.4 1 . 9 5 4 . 4 11.4h 2.3 5 . 2 1 3 . 0 . . . 5 . 4 13.0 2 . 9 6 . 5 14.0 2.1 5.4 2 . 8 6 . 6 16:s . . . 8 . 6 19.5
2.1
... ...
2.3 2.1 2.6
...
2.3 2.7
...
120 100 140 3 2
4.5 4.5 5.1 5.1 8.0 5.7 8.0 15.7
6 . 3 24 6.0 ... 6.5 6 . 5 l8:O 5 . 2 12.5
6 . 1 15.5 . . . . . . 15.5 . . . 14.5 . . . 9 : 5 22 55 3 . 6 7 . 4 15.5
6.5 7 . 0 20:Oh 6.5 6 . 0 1410
7.3 7.3 7.8 6.7
18 18 19.5 11.0
Diphenylformamide 60 1.65 1.40 2.4
100 100
140
3 . 7 10.2 3 . 5 8.7h 5 . 7 15.3
. . . . . .
2.95
6.5 6.7 4 . 4 10.7 3.1 7.7 4 . V 13.0 . . . 20.7 ,..
16.5 17.2 22.3 20h 33h 54.0
5.8h 16.3 . . . . . . 15.5 . . . . . . 15.5 . . . 7 . 0 20.0 62 3.4 8 . 5 22 6.6 6.6 7.3 5.6
7.0 7.0 10.0 10.0 11.2 11.2 17.0 12.0 17.0 33.0
3.2
17 ... 17 42h 21 ... 1 2 . 0 25h
46h 46h 4gh 21h
Tide0 60
100 100
1.60 1.60 1.60 1.80 2.30 2.30 2.30 1 80
4.1 4.1 4.1 4.4 5.3 5.3 5.3 49
140
12.3 12.3 12.3 13.0 14.0 14.0 14.0 13.5
Same as paraffins . . . 6 2 16.5
. . . . . . . . . . . . 6.2 ...
5.3 4 . 3 io.3 1 . 7 0 4 . 3 12.7
i.ko
1.95 4 . 7 1.95 4 . 7 14h' 2.00 5 . 3 . . .
Same as ethers
Table V. Observed Retention Volumes for n-Pentane for Various Substrates
(Column Size. 1 meter, tubing)
Substrate Convoil-20 2-Ethyl hexyl sebacate Convachlor-12 Fluorolube-S Dipropionitriles Fluorochemical N-43 (perfluorotribut ylamine) Squalane Apiezon-L Silicone, D.C. 703 Tricresyl phosphate Diphenylformamide Polypropylene glycol (400) Polypropylene glycol (2000) Tide Triethylene glycol Triethylene glycol (saturated with AgN03)
Temp., "C. 100
77
100 100 100
24 19 17 8
52
(52' C.)
inch metal Retention Volume, M1. at 25O c., 1 Atm.
67
Figure 8. Perfluor o t r i butylamine Naphthenes have lower retention than paraffins
11
100
100 100
82 26 61
100
34
100
15
120
19
120 100 67
20 12 12
67
4 RELATIVE
RETENTION (n-PENTANE 4 )
RELATIVE
RELATIVE
Figure 9.
RETENTION ( n - PENTANE = I )
Fluorolube-S ( 1 00' C.)
Relatively nonselective for hydrocarbons
Figure 10.
RETENTION
In-PENTANE - 1 )
Effect of silver nitrate in triethylene glycol Temperature 65' C. Triethylene glycol -Saturated with A g N 0 3
---
VOL. 30, NO. 1 , JANUARY 1958
0
7
Table VI.
Type of Separation Hydrocarbons Nonselective boiling point type of separation Paraffins from aromatics Naphthenes from aromatics Naphthenes from paraffins Paraffins from olefins Olefins from naphthenes Diolefins from olefins Cyclo-olefins from olefins Cyclopentanes from cyclohexanes Type I from Type I11 .. .. olefins Oxygenated Compounds Nonselective boiling- *Doint type separation Alcohols from other oxytypes Ethers from other oxytypes Primary, secondary, and tertiary alcohols from each other Ketones from esters Ketones from aldehydes Ketones from ethers a
Column" T."mz.,
Ratio of Retentions a t 100' C. B.P.
Squalane
100
Max. approx. 1 . 1
Dipropionitriles Dipropionitriles Dipropionitriles Dipropionitriles Diphenylformamide Di ropionitriles Poyystyrene oxide None
67 67 67 67 IO0 67 120
19 9.5 1.9 2 1.1 1.7 2.1
Triethylene glycol saturated with AgNOs
67
1.5
Polypropylene glycol P2000 Silicone D.C. 703
120
Max. approx. 1 . 3
Best Substrate Found
Dipropionitriles Convoil-20 or Convachlor12 Dipropionitriles Dipropionitriles Dipropionitriles
...
...
1 . 7 (min.)
100 67
5 (min.)
100
1:1.3:1.7
67 67 67
1.5 1.2 7
Not necessarily optimum.
the charts it will be concluded that when the retentions for two types differ by a factor of 2, their separation can be accomplished in a mixture having about a 25" C. boiling range. If the boiling range is accurately known, the analysis for the two types should be possible without knowledge of the actual compounds present. Figure 10 shows specifically the effect of silver nitrate dissolved in triethylene glycol for the three hydrocarbon typesparaffins, Type I olefins (CHFCH-R), and alkylbenzenes. Silver ion affects the relative retention of only the Type I olefins. The actual retention volume for the n-pentane reference is reduced by a factor of 3, as indicated in Table V. Separations of Oxygenated Types. T h e d a t a of Table VI indicate rather easy separation of ethers from other oxygenated types over a n extended boiling range b y t h e dipropionitriles. A polypropylene glycol of high molecular weight (P-2000) appears t o be best for a nonselective boiling point type of separation of oxy compounds. A nonpolar liquid substrate favors the elution of alcohols first and ethers last. A polar material tends to reverse this order, alcohols or ketones being eluted last. The limited data likewise indicate a reversalin the order of elution of alcohol types: primary, secondary, tertiary on Convoil-20 with a reversal of this order on the dipropionitriles. More complete data are required t o demonstrate that this is generally true. A saturated hydrocarbon substrate would best serve for the separation of
8
Liquid Substrates for Separations
ANALYTICAL CHEMISTRY
minor amounts of oxygenated compounds from hydrocarbons in a mixture of limited boiling range. Acetylenes would be difficult t o separate from ethers. T o identify low concentrations of hydrocarbon in mixtures of oxygenated compounds, the dipropionitriles appear to be best. However, acetylenes and ethers are difficult t o separate and alkylbeneenes would overlap several oxygenated types.
CONCLUSIONS
Eighteen different liquid substrates have been screened as to their selectivity toward types of hydrocarbons and oxygenated compounds. For hydrocarbons, highly selective materials are available for separation of aromatics from saturated compounds. Cyclic hydrocarbons may be separated from acyclic compounds in a mixture having a boiling point spread of 20" to 2.5" C., for both paraffins and olefins. Something better is desirable, and something much more selective is needed to separate acyclic olefins from equivalent boiling naphthenes and diolefins from acyclic olefins. Selectivity toward cyclopentanes us. cyclohexanes may involve wishful thinking. Separation of ethers from other oxygenated types (except acetals) looks feasible for a mixture boiling range of 80" to 90" C. Alcohols may be separated from other types over a boiling range of about 15" C. Ketones may be
separated reasonably well from esters but not from aldehydes. The dipropionitriles have the highest selectivity of those materials tested, for both hydrocarbon and oxygenated compound types.
LITERATURE CITED
(1) Desty, D. H., Whyman, B. F. H., ANAL. CHEM.2 9 , 320 (1957). (2) James, A. T., Martin, A. J. P., J.App2. Chem. 6,105 (1956). (3) James, A. T., Martin, A. J. P., Smith, G. H., Biochem. J . (London)52, 238 (1952). (4) Keulemans, A. J. M., Kwantes, A., Zaal, P., Anal. Chim. Acta 13, 357 (1955). ' (51 Purnell. J. H.. Proceedings of Svmposium on Vapor Phase chromatography, London, May 30-June 1, 1956, p. 52, Butterworths Scientific Publications, London, 1957. (6) Simmons, M. C., Snyder, L. R., Division of Petroleum Chemistry, 131st Meeting, ACS, Miami, Fla., April 1957. (71 ~, Tennev. H. M., Harris, R. J., ANAL. CH~M. 2 9 , 317 (1957): (8) Tenney, H. M., McQuaid, F. S., McQuaid,[J.i W., Southwide Chemical Conference, ACS, Memphis, Tenn., Dec. 6-8, 1956. \ - I
RECEIVED for review June 3, 1957. Accepted September 20, 1957. Divisions of Analytical and Petroleum Chemistry, Symposium on Progress in Gas Chromstogrephy, 132nd Meeting, ACS, New York, N. Y., September 1957.
